1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine and a printer utilizing the electrophotographic process or the electrostatic recording process, and more particularly to an image forming apparatus capable of forming an image in a monochrome mode and a full-color mode.
2. Description of the Related Art
A multicolor electrophotographic copying machine for forming a full-color image with the electrophotographic process will be described below by way of example.
In the multicolor electrophotographic copying machine, a series of toner image forming processes, i.e., charging, exposing and developing processes, are carried out on a photoconductive drum serving as an image carrier to form toner images of four colors, i.e., a yellow toner image, a cyan toner image, a magenta toner image and a black toner image, which are color-separated toner images of a full-color image. Those toner images are transferred onto the same recording medium in superimposed relation and then fused under heating for permanent fixation, whereby the full-color image is formed by mixing of the four colors.
As a means for charging the photoconductive drum, a contact charging device is used because of such advantages that it is of the low-voltage type and generates less ozone.
The contact charging device comprises an electrically conductive member (contact charging member) in the form of a roller, for example, which is disposed in contact with the photoconductive drum. A charging bias is then applied to the contact charging member, whereby the surface of the photoconductive drum is charged to have a predetermined polarity and potential.
Contact charging is divided into a DC charging method in which the surface of the photoconductive drum is charged by applying, as the charging bias, only a DC voltage to the contact charging member, and an AC charging method in which the surface of the photoconductive drum is charged by applying an AC voltage component and a DC voltage component to the contact charging member. The AC charging method is superior to the DC charging method in stability against environmental changes and uniformity of charging.
The multicolor electrophotographic copying machine described above forms a full-color image by reproducing a full color by combinations of four-color toner images, i.e., a yellow toner image, a cyan toner image, a magenta toner image and a black toner image. Accordingly, if uniformity of a surface potential of the photoconductive drum is deteriorated due to, e.g., variations in charging, an image failure such as unevenness in image color would occur.
Because the DC charging method has a difficulty in keeping uniformity of charging, the AC charging method is essential and used to perform the contact charging of the photoconductive drum in the multicolor electrophotographic copying machine described above.
The multicolor electrophotographic copying machine having the above-described construction operates in a very satisfactory manner, but has the following problems.
An AC component applied for the contact charging in the AC charging method tends to destroy the bridges of a high-molecular material, e.g., polycarbonate, which forms the surface of the photoconductive drum, and therefore the photoconductive drum surface is likely to become brittle. As shown in FIG. 8, therefore, the AC charging method causes an abrasion of a surface layer of the photoconductive drum per rotation of the drum 2 to 3 times as large as that with the DC charging method.
Thus, looking at the photoconductive drum from the viewpoint of useful life, the AC charging method is one factor which shortens the useful life of the photoconductive drum.
Meanwhile, with widespread color copying or printing of business documents, etc., color copying machines and color printers have been recently employed in offices and the likes. Because installing both a full-color copying machine or printer and a monochrome copying machine or printer is disadvantageous from the cost point of view, a full-color copying machine or printer is equipped to be also used as a monochrome copying machine or printer in many cases.
In such a case, a usage rate of monochrome output is usually higher than that of full-color output. It is estimated that the usage rate of monochrome output is 60% at minimum and approximately 95% at maximum.
Under the above situation, the running cost during the monochrome output operation is very important. A rate of the cost of the photoconductive drum in the total running cost is great. In other words, a reduction in useful life of the photoconductive drum resulted from employing the AC charging method increases the running cost during the monochrome output operation.
Accordingly, it is an object of the present invention to provide an image forming apparatus which can produce a full-color image with high quality.
Another object of the present invention is to provide an image forming apparatus in which an image carrier has a longer useful life.
Still another object of the present invention is to provide an image forming apparatus which includes an image carrier for carrying an electrostatic latent image, a charging member contacting the image carrier and charging to the image carrier with electricity, a voltage applying unit for applying a voltage to the charging member, a selecting unit for selecting one of a monochrome mode for forming an image in a single color and a full-color mode for forming an image in multiple colors, and a changing unit for changing a voltage applied from the voltage applying unit to the charging member between the monochrome mode and the full-color mode.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.